1. Field of the Invention
The present invention relates to a magnetic flux detecting apparatus of a flux locked loop (FLL) system using a superconducting ring having a Josephson junction, i.e., a superconducting quantum interference device (SQUID).
2. Description of the Related Art
An FLL system generally carries out measurement of a magnetic field by feeding a feedback current to a feedback coil additionally provided at the SQUID so that a value of a first defined lock point is maintained or so that a change rate of a magnetic flux in a superconducting ring becomes always constant. That is, in order to eliminate an external magnetic field, a feedback current is fed so that a magnetic field reversal from the external magnetic field is generated, and then, a change rate of the external magnetic field is obtained by a magnitude of this feedback current. By using an FLL apparatus, linearity data can be sampled such that an external magnetic flux to be measured and an output voltage that is a measurement value of this external magnetic flux lie in a proportional relationship.
As such a system, there is proposed a magnetic flux detecting method using a so called digital FLL system. In the digital FLL system, periodic property of Φ−V characteristics of the SQUID is utilized. A large change component of a magnetic flux is measured by counting the number of periodic changes in the Φ−V characteristics, and then, a small change component of the magnetic flux is linearly measured and merged. A measuring portion of the SQUID FLL apparatus is primarily configured using an AD converter, a digital integrator, a counter, a DA control converter, and a control measurement computer. In this case, in order to achieve a high resolution and a high slew rate, there is a need for expensive circuit components having a large number of processing bits and enabling a high speed processing operation. Therefore, the circuit components are formed in a digital signal processing (DSP) 22.
In such a digital FLL magnetic flux detecting method, using one counter, external magnetic flux data is expressed by merging a value expressing a magnetic flux from the number of bits based on data from the AD converter and the number of bits expressing the number of period from the counter.
FIG. 1 shows a dcSQUID magnetometer 10 using a digital FLL technology. A SQUID 11 provides two Josephson junctions 13 partway of a ring 12 made of a superconducting material, and is biased by means of a direct current Ib from a direct current power supply (not shown). Then, a voltage (output voltage V) between an input and an output of this bias current is changed by an external magnetic flux Φx that penetrates the ring 12 of the SQUID. FIGS. 2(a) and 3(a) each show a relationship between the external magnetic flux Φx and the output voltage V. The output voltage V of the SQUID 11 periodically changes in accordance with a change of the external magnetic flux Φx that penetrates the ring 12. Its period is Φ0 that is a magnetic flux quantum. In this way, the output voltage V periodically changes, and thus, a value of the external magnetic flux Φx is not uniquely defined merely by measuring an output voltage V.
Thus, as shown in FIG. 2(a), there is used a method for carrying out measurement including a periodic change of a magnetic flux from an arbitrary measurement start point “a0” (generally called “lock point”). That is, there is employed a system of calculating a value of the external magnetic flux Φx based on the number “n” of periodic changes based on the external magnetic flux and a change component Φ′ of the magnetic flux in a maximum period an. In general, each lock point is defined at a point of a voltage equal to another one for each period. This lock point can be arbitrarily set in accordance with convenience of a data processing operation, and it is not always necessary to be V=0, as illustrated in FIG. 2.
In order to measure values that correspond to periodic property and a change component Φ′, as shown in FIG. 2B, a change component Δv of an output voltage is obtained, the Δv corresponding to a magnetic flux change component ΔΦ from a lock point an of a certain moment, and then, the thus obtained change component is always fed back to a feedback coil 20 via an integrator circuit. Thus, a measurement point is fixed to the lock point an, and the change Δv of the output voltage based on the change ΔΦ of the magnetic flux at the time of each measurement becomes always constant. Thus, as shown in FIG. 2C, a voltage change component V′ corresponding to the change component Φ′ of the magnetic flux can be obtained as linearity data. If this data value exceeds a control range of a lock point, the current lock point moves to a next lock point, and at the same time, previous integral data in the integrator is reset.
In an example of FIG. 1, an output voltage V of the SQUID 11 is amplified by means of an amplifier 14, and the thus amplified output voltage is converted to digital data by means of an AD converter 15. The digital data is integrated by means of a digital integrator 16. If an integral value exceeds a control range of each lock point, the value is reset. In accordance with the reset count, up to what period of data is obtained is measured by means of a counter 17. An integral value of each period is fed back to the feedback coil 20 via a voltage/current converter 19 for generating a feedback current “If” defined in response to characteristics of a DA converter 18 and the SQUID 11. In addition, each integral value reset by lock points (a0, a1, a2 . . . an) of each period is fed to a data merge unit 21. The feedback current “If” is reset for each period, and thus, does not increase to a predetermined value or more.
The data merge unit 21 calculates a value of a magnetic field that corresponds to the reset count measured by the counter 17 and a value of the magnetic field that corresponds to a voltage change component V′ obtained from the digital integrator 16 in a last period, sums these values, and then, obtains the value of the external magnetic flux. Control of the AD converter 15, the digital integrator 16, the counter 17, and the data merge unit 21 is generally carried out by means of a control unit (not shown) of the DSP 22.
In addition, in the digital FLL, the control range of lock points is defined as ±1Φ0 of control lock points (a0, a1, a2 . . . an), as shown in FIG. 3(b). Then, in the case where the magnetic flux Φ has exceeded this range, there is used a method for shifting a lock point, recording UP and DOWN information by means of the counter 17, and then, carrying out control (feedback). In this method, stabilization of an operation of switching a control range is attempted by utilizing so called hysteresis characteristics in which lock point and voltage change paths are different between cases in which an external magnetic flux increases and decreases.
Non-patent document 1: Dietmar Drung “HIGH-Tc and low-Tc dc SQUID electronics” Superconductor Science and Technology 16 (2003) 1320-1136